Quite the combination. I'll jump right into it.
Many players look at Silver Keys and have the brief thought of "wow, I wish I could afford that" before moving on. Additionally, some players open Lockboxes only to find another Spike Mohawk, and would really rather that they get their crowns' worth from the boxes.
My solution to both is a variety of Key types. At their most basic, they would consist of three; Bronze, Silver, and Brass/Gold.